An inductor is an electronic device used in circuits for its property of inductance. The behavior of inductors is related to phenomena associated with magnetic fields. When electric current flows through an inductor, magnetic fields may be created. Moreover, if the current varies with time, then the magnetic field will vary with time. This time-varying magnetic field induces a voltage in electrical conductors exposed to it. The circuit parameter of inductance relates the induced voltage to the current.
One property of an inductor is its quality factor, Q. The quality factor of an inductor is proportional to the ratio of its inductance to its resistance at a given frequency, and is a measure of the inductor's efficiency. The higher the quality factor, the more efficient the inductor is. The quality factor of an inductor may be influenced by several factors, one factor being eddy currents, which are the circulating flow of charges caused by a moving magnetic field within a nearby conductive material or device. The flow of eddy currents generates magnetic fields that oppose changes in external magnetic fields. Generation of eddy currents near an inductor degrades the inductor's quality factor, and thus if one is concerned with the quality factor of an inductor, it is usually desirable to avoid having eddy currents in devices near inductors.
On-chip inductors, despite being large, are often made of only two metal layers. A minimum metal density requirement for each metal layer over an entire wafer of an IC may be set to reduce topographical variations, increase uniformity, and target a certain yield. Therefore, it may be desired to increase the metal density count of a layer. Placing metal fills inside or near an inductor increases the metal density count, but has the possible drawback of decreasing the quality factor of the inductor through eddy current loss. Measurement results of inductors with metal fills have been reported before, but none have established a relationship between fill cell size and the inductor quality factor. This relationship is of great interest because it allows one to estimate the largest device that can be placed inside an inductor.
It should be noted that the topmost metal layer in a fabrication process is often referred as the thick top metal. The thick top metal layer is often several times thicker than the other metal layers, and thus has a lower resistance. This is beneficial for making inductors because a lower resistance improves the quality factor of an inductor.
Spiral on-chip inductors are a type of inductor used in the design of, for example, radio frequency integrated circuits (RFICs). These spiral on-chip inductors often occupy more than half of the total chip area in RFICs. The region in and around an on-chip inductor is typically kept clear of active and passive devices to avoid the generation of eddy currents in the devices, which, as discussed, degrades the quality factor of the inductor. However, leaving the area near an inductor empty is a waste of space and increases chip size. This is a problem because reducing the area, and therefore the cost of circuits is a concern in circuit design.
Therefore, it is beneficial to reduce restrictions on the spacing among devices on a circuit. Specifically, it would be of benefit to reduce spacing restrictions between inductors and other devices.